This invention relates to catoptric systems, and more particularly to an objective lens system requiring a large relative aperture ratio such as that employed in cameras for X-ray fluoroscopic photography.
A typical example of the prior art objective lens system of the character described is disclosed in Japanese patent No. 39-26447, and is illustrated in FIG. 1. This catoptric objective lens system may be considered as comprising three components of different function, including a first component, counting from the side of the longer conjugate, which is composed of a positive meniscus lens element I.sub.1 and a negative meniscus lens element I.sub.2 spaced apart from each other and contributes a weak diverging power to the complete system, a second component, which functions only as a reflecting means, and a third component, which includes a negative lens element but contributes a converging power to the complete system. The aberrations, such as spherical aberrations and coma of the complete system, other than the curvature of the image field resulting from the use of the mirror II are corrected by a suitable design of the first component I, and the curvature of the image field is compensated for by a suitable design of the third component III. In order to compensate for the spherical aberrations resulting from the use of the mirror II, a divergent lens component must be selected for employment as the first component I with sacrifice of minimization of the mirror diameter.
On the other hand, from the point of view of facilitating aberrational correction, the basic structure of the prior art high aperture catoptric objective lens system is disadvantageous in effecting sufficiently well correction for outward coma aberration resulting from the use of the mirror, because the positive meniscus lens element I, which serves to correct the coma aberration, and the negative meniscus lens element I.sub.2, which serves to correct the spherical aberrations, are arranged in closely spaced relation to each other in the first component I to thereby produce an adverse effect on the correction for the outward coma aberration.